This program project serves as the nucleus for the study of the biological aspects of aging at Boston University. The primary objective is to learn why animals age. Three systems are studied, the auditory system, the brain, and connective tissue. The research on the auditory system involves measurements of the actual hearing of rats, examination of their cochlea, and their brains. The brain studies also include other cortical areas and the olfactory bulb. The connective tissue studies are primarily of elastin synthesis and maturation in both intact vessels and by the cells of blood vessel wall in tissue culture. Under the scanning electron microscope there is loss with age of both receptor cells and inner and outer hair cells of the organ of Corti. In the larger II and V pyramidal cells there is a diminution in the number of spines along the apical and basal dendrites and a tendency for both the cell bodies and the nucleus to be smaller in older animals. Studies in the olfactory bulb have produced the rather surprising finding that the number of granule cells increases with age even in adult animals. While H1 histone could be detected in the polysomes of young liver it could not be found in the polysomes of young brain. The cytosol of brain seems to contain instead a protein resembling Chalkley's Fo, a histone-like protein which increases with age in older tissue's nuclei.